Deflector devices

ABSTRACT

A deflector device for use with a tow line between a seismic survey vessel and a tow, in particular a streamer or streamer array, in the water behind the vessel comprises a vertically oriented wing-shaped body for producing a sideways force as it is towed through the water, and a towing bridle adapted to connect the wing-shaped body to the tow line. The bridle comprises first and second connecting elements connected between the tow line and respective longitudinally-spaced points along the high pressure side of the wing-shaped body. The wing-shaped body includes one or more buoyancy elements to render it slightly positively buoyant, and the length of at least one of the connecting elements is adjustable by remote contol in order to tilt the wing-shaped body. This gives the sideways force a vertical component, and so allows remote control of the depth of the deflector device, as well as its lateral offset from the vessel.

[0001] This invention relates to deflector devices of the kind usedbetween a towing vessel and a tow located in water, for example aseismic streamer or streamer array, or a seismic source array, in orderto pull the tow out to one side of the vessel, so as to position it at adesired lateral offset from the course followed by the vessel.

[0002] A deflector device of this kind is described in detail in ourU.S. Pat. No. 5,357,892, and comprises a wing-shaped deflector bodyhaving a remotely-operable pivotal lever or “boom” which extendsrearwardly from a point near the middle of the trailing edge of thewing-shaped body. In use, the wing-shaped body is suspended beneath afloat so as to be completely submerged and positioned generallyvertically in the water, and is connected to the towing vessel by meansof a tow Line, while the tow is connected to the end of the boom remotefrom the wing-shaped body. As the device is pulled through the water,the wing-shaped body produces a sideways force, or “lift”, which movesthe tow laterally. This lift can be varied by adjusting the angle of theboom from the vessel, thus pennitting the lateral offset of the tow fromthe course of the vessel to be varied in use.

[0003] The deflector device of U.S. Pat. No. 5,357,892 has beensuccessfully commercialised by the Applicant as its MONOWING deflectordevice. In use, rolling stability of the device is provided by theconnection to the float, while stability of the device about a verticalaxis is provided by the drag produced by the tow.

[0004] The MONOWING deflector devices in current use are very large,typically 7.5 m high by 2.5 m wide, and weigh several tonnes. They areusually suspended around 2 m to 8 m below the float by means such as afibre rope, and are also provided with a safety chain intended toprevent separation of the float and wing-shaped body in the event thatthe rope breaks. In rough weather, the upper part of the wing-shapedbody may rise up out of the water, allowing the rope connecting thewing-shaped body and the float to go slack. If the wing-shaped body thendrops abruptly, the rope, and possibly even the safety chain, may break,and/or their attachment points on the wing-shaped body may be badlydamaged.

[0005] Additionally, the depth at which the current deflector deviceoperates is effectively determined by the length of the rope connectingit to the float. As a result of this, the operating depth of thedeflector device cannot readily be varied while the device is deployedin the water. And since the normal operating depth of the currentdeflector device is typically a few meters, in the event of the onset ofbad weather, the device and all the streamers and other equipmentdirectly or indirectly attached to it have to be recovered onto thetowing vessel.

[0006] It is an object of the present invention to alleviate thedrawbacks arising from the connection of the deflector device to thefloat.

[0007] According to the present invention, there is provided a deflectordevice for use with a tow line between a towing vessel and a tow inwater behind the vessel, the device comprising a wing-shaped body, and atowing bridle adapted to connect the wing-shaped body to the tow line,the bridle comprising first and second connecting elements havingrespective first ends connected to respective longitudinally-spacedpoints along the high pressure side of the wing-shaped body andrespective second ends adapted to be coupled to the tow line, and thewing-shaped body being shaped to produce in use a sideways force whichurges the tow line laterally with respect to the direction of movementof the towing vessel, further comprising one or more buoyancy elementsdisposed within and/or secured to the upper end of the wing-shaped body,and remotely-operable means for adjusting the length of at least one ofthe connecting elements in order to tilt the wing-shaped body so as togive said sideways force a vertical component, whereby to control thedepth of the deflector device as well as its lateral offset from thevessel.

[0008] It will be appreciated that since the deflector device of theinvention can generate a controllable vertical force, this force,together with the buoyancy of the one or more buoyancy elements, can beselected and adjusted so that the separate surface float is no longerrequired, and the operating depth of the device can be remotelycontrolled while the device is deployed in the water. In particular, atthe onset of bad weather, the deflector device and its tow can be causedto dive to a greater depth, where the effects of the bad weather aremuch reduced, until the weather improves.

[0009] Advantageously, the one or more buoyancy elements has or have abuoyancy selected to give the complete device a small positive buoyancy.

[0010] The remotely-operable adjusting means preferably comprises atelescopic member, which may be hydraulically actuated, connected inseries in one of the connecting elements, which are advantageouslytitanium chains.

[0011] In a first implementation of the invention, the deflector devicefurther comprises a boom extending rearwardly from the wing-shaped body,the end of the boom remote from the wing-shaped body being connected, inuse, to the tow, and remotely-operable means for adjusting the anglebetween the boom and the wing-shaped body to vary the sideways forceproduced by the wing-shaped body.

[0012] In a second implementation of the invention, the deflector devicefurther comprises a boom extending rearwardly from the wing-shaped body,an auxiliary wing-shaped body, smaller than the firstmentioned (orprincipal) wing-shaped body, secured to the end of the boom remote fromthe principal wing-shaped body and shaped so as to produce in use asideways force in generally the opposite direction to that produced bythe principal wing-shaped body, and remotely-operable means foradjusting the angle between the boom and the principal wing-shaped bodyto vary the sideways force produced by the principal wing-shaped body.

[0013] In a third and preferred implementation of the invention, thedeflector device further comprises a boom extending rearwardly from thewing-shaped body, an auxiliary wing-shaped body, smaller than thefirstmentioned (or principal) wing-shaped body, secured to the end ofthe boom remote from the principal wing-shaped body and shaped so as toproduce in use a sideways force in generally the opposite direction tothat produced by the principal wing-shaped body, and remotely-operablemeans for varying the angle of the auxiliary wing-shaped body to varythe sideways force produced by the auxiliary wing-shaped body, andthereby vary the sideways force produced by the principal wing-shapedbody.

[0014] Advantageously, the auxiliary wing-shaped body is provided with atrailing edge flap angled away from the boom, typically at about 35°.

[0015] The invention also includes a method of performing a marineseismic survey, the method including towing a plurality of laterallyspaced seismic steamers over an area to be surveyed, wherein the lateralposition and the depth of at least one of the streamers are controlledby a deflector device in accordance with any one of the precedingstatements of invention.

[0016] The invention will now be described by way of example only, withreference to the accompanying drawings, of which:

[0017]FIG. 1 is a somewhat schematic view of a seismic survey vesselcarrying out a marine seismic survey;

[0018]FIG. 2 is a somewhat schematic part-sectional view of a firstembodiment of a deflector device in accordance with the presentinvention, for use in carrying out the survey of FIG. 1;

[0019]FIGS. 3A and 3B are respective perspective views of the deflectordevice of FIG. 2;

[0020]FIG. 3C is a more detailed view of part of the deflector device ofFIG. 2:

[0021]FIG. 4A is a somewhat schematic part-sectional view of a secondembodiment of a deflector device in accordance with the presentinvention, for use in carrying out the survey of FIG. 1; and

[0022]FIGS. 4B and 4C show different operating positions of part of thedeflector device of FIG. 4A.

[0023] The seismic survey vessel shown in FIG. 1 is indicated generallyat 10, and is preferably as described in our PCT Patent Application No.PCT/GB98/01832 (WO 99/00295). The vessel 10 is shown towing a seismicsource 15, typically a TRISOR multiple air gun source of the kinddescribed in our U.S. Pat. No. 4,757,482, and an array 16 of foursubstantially identical streamers 18. However, it will be appreciatedthat, in practice, many more than four streamers can be towed, forexample by using the techniques described in our PCT Patent ApplicationNo. PCT/IB98/01435 (WO 99/15913). The streamers 18 are towed by means oftheir respective lead-ins 20 (ie the high strength steel- orfibre-reinforced electrical or electro-optical cables which conveyelectrical power, control and data signals between the vessel 10 and thestreamers), and their spread is controlled by two deflector devices,indicated at 22, connected to the respective forward ends 24 of the twooutermost streamers. The deflector devices 22 act in co-operation withrespective spreader lines 26 connected between the forward end 24 ofeach outermost streamer 18 and the forward end 24 of its adjacentstreamer to maintain a substantially uniform spacing between thestreamers.

[0024] One of the deflector devices 22 is shown in section in FIG. 2.The deflector device 22 is similar in general principle to the deflectordevice of our U.S. Pat. No. 5,357,892, but is a much improved version ofit. In particular, the deflector device 22 has a main wing-shaped body28 which is coupled in use to a respective outer lead-in 20, and whichcorresponds to the deflector body 2 of U.S. Pat. No. 5,357,892. However,the main wing-shaped body 28 is of improved hydrodynamic cross-sectionalshape and includes a fixed-angle trailing edge flap 29, both of whichfeatures enhance lift. Also, the main wing-shaped body 28 is providedwith vortex controlling end plates 30 (see FIGS. 3A and 3B) of the kinddescribed in our PCT Patent Application No. PCT/FR99/02272, to reducedrag and improve stability, and is largely made of titanium to reduceweight.

[0025] Additionally, the angle lever 10 of U.S. Pat. No. 5,357,892 isreplaced by a rearwardly extending fixed angle boom 32, which isdetachably connected at one end 34 to the low pressure side 36 of thebody 28 near the trailing edge flap 29, at a mounting bracket 38. Theboom 32 is of sandwich construction, and is made from two similarlyshaped plates 39 which are bolted together at intervals along theirlength and which sandwich between them the mounting bracket 38.Typically, the boom 32 is detached whenever the deflector device 22 ison the vessel 10, for ease of stowage. The other end 40 of the boom 32has a towing eye 42, coupled in use to the forward end 24 of arespective one of the two outermost streamers 18.

[0026] An auxiliary wing-shaped body 44, which is much smaller than thebody 28 in length, thickness and chord, is. pivotally secured as will beexplained hereinafter to the end 40 of the boom 32, with itslongitudinal axis (which lies in a plane perpendicular to the plane ofFIG. 2) extending parallel to the longitudinal axis of the body 28. Theshape of the body 44 is designed to produce, in use, a sideways force ina direction approximately opposite to that produced by the body 28(approximately opposite, because as will become apparent, the directionof the force varies in use). This sideways force is increased byproviding the body 44 with a fixed trailing edge flap 46, angled awayfrom the boom 32 at an angle of about 35°.

[0027] As best seen in FIGS. 3A and 3B, the auxiliary wing-shaped body44 is implemented in two symmetrical halves 44 a and 44 b, which eachhave vortex-reducing end plates 45 and which are disposed on oppositesides of the boom 32. The two halves 44 a, 44 b of the auxiliarywing-shaped body 44 are rotatable in unison about a common axisperpendicular to the plane of the boom 32, so as to vary the angle ofthe chord of the auxiliary wing-shaped body 44 with respect to the boom.Rotation of the auxiliary wing-shaped body 44 is effected by atelescopic actuator 48 pivotally mounted between the plates 39 of theboom 32, the actuator being pivotally connected to a lever arm oreccentric 47 attached to the two halves 44 a and 44 b of the auxiliarywing-shaped body 44 (see FIG. 3C). The telescopic actuator 48 isoperated from a remotely-controllable electro-hydraulic control pack 49,which is also mounted between the plates 39 of the boom 32.

[0028] It will be appreciated that varying the angle of the auxiliarywing-shaped body 44 of the deflector device 22 changes the angle of themain wing-shaped body 28 with respect to the direction of tow, and sochanges the lift produced by the main wing-shaped body. This in turnchanges the lateral offset produced by. the deflector device 22.

[0029] In accordance with the present invention, the deflector device 22is made approximately neutrally buoyant, by including gas-filledpipe-like buoyancy elements 58 extending longitudinally within it fromtop to bottom, and/or by providing an integral buoyancy element at itsupper end similar to but smaller than that described in our co-pendingUnited Kingdom Patent Application Nos. 0023775.0, 0025719.6 & 0029451.2.In practice, the deflector device 22 is preferably designed to beslightly positively buoyant, so that in the event of a malfunction, ittends-to-float-rather than sink. Additionally, the main wing-shaped body28 of the deflector device 22 is coupled to the respective lead-in 20 bya towing bridle 50 comprising two titanium chains 52 and 54, the chain54 having a remotely operable, hydraulically actuated, telescopic strut56 connected in series in it.

[0030] With the telescopic strut 56 in its mid-length position, thecombined length of the chain 54 and the strut 56 is substantially equalto the length of the chain 52, which tends to hold the main wing-shapedbody 28 in a substantially vertical attitude in the water, so thatsubstantially all the force or “lift” generated by it is directedsideways, as in the prior art MONOWING deflector device, but with justenough of a downward component to counteract the slightly positivebuoyancy mentioned above. However, changing the length of the strut 56tends to tilt the main wing-shaped body 28 away from the vertical, sogiving the sideways force generated by it a more significant verticalcomponent in the upward or downward direction, and thus permitting thedepth of the device to be varied.

[0031] It will be appreciated that as a result of making the deflectordevice 22 approximately neutrally buoyant and capable of generating aremotely-controllable vertical force, a separate surface float is nolonger required, and the operating depth of the device can be remotelycontrolled while the device is deployed in the water. In particular, inthe event of the onset of bad weather, the deflector device 22 and thestreamers 18 attached to it can be caused to dive to a greater depth,where the effects of the bad weather are much reduced, until the badweather passes.

[0032]FIGS. 4A to 4C show at 60 an alternative embodiment of thedeflector device 22 of FIGS. 2 and 3A to 3C, with corresponding partshaving the same reference numbers as were used in FIGS. 2 and 3A to 3C.The principal difference between this alternative embodiment and theembodiment of FIGS. 2 and 3A to 3C is that in the deflector device 60,the boom 32 is pivotally connected to the low pressure side 36 of themain wing-shaped body 28 at the mounting bracket 38, while the auxiliarywing-shaped body 44 is fixedly secured at or near the midpoint of itstrailing edge 62 to the end 40 of the boom 32, with its leading edge 64inclined away from the body 28 such that the chord of the body 44 isinclined at an angle of about 10° to the boom.

[0033] Pivotal movement of the boom 32 is controlled by a mechanismcomprising first and second struts 66, 68, which are pivotally connectedto each other at 70 and to each end of the boom at 71 a and 71 b,forming with the boom a triangle, and an extending hydraulic actuatorstrut 72 pivotally connected between the apex of the triangle, ie thepivotal connection point 70 of the struts 66, 68, and a pivotalconnection point 74 positioned on the low pressure side 36 of the body28 between its midpoint and its trailing edge. The actuator strut 72 isconnected to be operated by a remotely-operable hydraulic control system(not shown) disposed within the body 28.

[0034] It will be appreciated that extension of the hydraulic actuatorstrut 72, from its unextended position of FIG. 4A, will move the boom 32outwardly from the low pressure side 36 of the body 28, from its closestposition shown in FIG. 4A. The extent of the outward movement ispreferably about 209, as shown in FIGS. 4B and 4C.

[0035] As the boom 32 is pivoted away from the body 28, the sidewaysforce produced by the body 44 acts as a restoring force, and thus variesthe angle of the body 28 with respect to the direction of tow, sochanging the lift produced by the body 28. This restoring force augmentsthe restoring force produced by the drag of the towed streamer 18 (and.in particular, reduces the effect of any stability-reducing variationsor reductions in that drag). Indeed, the deflector device 60 will remainstable with no streamer attached, eg if its streamer 18 breaks or issevered at its forward end 24 (this is also true for the deflectordevice 22 of FIGS. 2 and 3A to 3C).

[0036] It will be appreciated that many modifications can be made to thedescribed embodiments of the invention.

[0037] In particular, the titanium chains 52, 54 of the towing bridle 50can be replaced by cables made from high strength fibres, eg Kevlarfibres, while the telescopic strut 56 can be replaced by any othersuitable hydraulic or electric mechanism for changing the relativelengths of the chains or cables, which mechanism can be housed insidethe body 28 and arranged to retract or pay out one or both of the chainsor cables. And the auxiliary wing-shaped body 44 can be made from aplastics material reinforced with high strength fibres, eg Kevlarfibres, and, in the deflector device 22, electrically operated ratherthan operated by the hydraulic actuator 48.

[0038] Additionally, the devices 22 and 60 can be used with tows otherthan streamers, for example seismic sources, and the tow need not beconnected to the end 40 of the boom 32 (it could instead be connected tothe lead-in 20, at a point near where the bridle 24 is connected to thelead-in). Also, the invention can if desired be used with a deflectordevice like that described in our U.S. Pat. No. 5,357,892, ie adeflector device without the auxiliary wing-shaped body 44.

[0039] Finally, although the invention has been described in relation todeflector devices whose lift can be varied by varying the angle of thedevice with respect to the direction of tow, it is also applicable inits broadest aspect to a fixed angle deflector device, eg of the kindreferred to as a “door”.

1. A deflector device for use with a tow line between a towing vessel and a tow in water behind the vessel, the device comprising a wing-shaped body, and a towing bridle adapted to connect the wing-shaped body to the tow line, the bridle comprising first and second connecting elements having respective first ends connected to respective longitudinally-spaced points along the high pressure side of the wing-shaped body and respective second ends adapted to be coupled to the tow line, and the wing-shaped body being shaped to produce in use a sideways force which urges the tow line laterally with respect to the direction of movement of the towing vessel, further comprising one or more buoyancy elements disposed within and/or secured to the upper end of the wing-shaped body, and remotely-operable means for adjusting the length of at least one of the connecting elements in order to tilt the wing-shaped body so as to give said sideways force a vertical component, whereby to control the depth of the deflector device as well as its lateral offset from the vessel.
 2. A deflector device as claimed in claim 1, wherein the one or more buoyancy elements have a buoyancy selected to give the complete device a small-positive buoyancy.
 3. A deflector device as claimed in claim 1 or claim 2, wherein the remotely-operable adjusting means comprises a telescopic member connected in series in one of the connecting elements.
 4. A deflector device as claimed in claim 3, wherein the telescopic member is hydraulically operated.
 5. A deflector device as claimed in any preceding claim, wherein the connecting elements are chains.
 6. A deflector device as claimed in claim 5, wherein the chains are titanium chains.
 7. A deflector device as claimed in any preceding claim, further comprising a boom extending rearwardly from the wing-shaped body, the end of the boom remote from the wing-shaped body being connected, in use, to the tow, and remotely-operable means for adjusting the angle between the boom and the wing-shaped body to vary the sideways force produced by the wing-shaped body.
 8. A deflector device as claimed in any one of claims 1 to 6, further comprising a boom extending rearwardly from the wing-shaped body, an auxiliary wing-shaped body, smaller than the principal wing-shaped body, secured to the end of the boom remote from the principal wing-shaped body and shaped so as to produce in use a sideways force in generally the opposite direction to that produced by the principal wing-shaped body, and remotely-operable means for adjusting the angle between the boom and the principal wing-shaped body-to vary the sideways force produced by the principal wing-shaped body.
 9. A deflector device as claimed in any one of claims 1 to 6, further comprising a boom extending rearwardly from the wing-shaped body, an auxiliary wing-shaped body, smaller than the principal wing-shaped body, secured to the end of the boom remote from the principal wing-shaped body and shaped so as to produce in use a sideways force in generally the opposite direction to that produced by the principal wing-shaped body, and remotely-operable means for varying the angle of the auxiliary wing-shaped body to vary the sideways force produced by the auxiliary wing-shaped body, and thereby vary the sideways force produced by the principal wing-shaped body.
 10. A deflector device as claimed in claim 8 or claim 9, wherein the auxiliary wing-shaped body is provided with a trailing edge flap angled away from the boom.
 11. A deflector device as claimed in claim 10, wherein the auxiliary wing-shaped body is provided with a trailing edge flap angled away from the boom at about 35°.
 12. A method of performing a marine seismic survey, the method including towing a plurality of laterally spaced seismic steamers over an area to be surveyed, wherein the lateral position and the depth of at least one of the streamers are controlled by a deflector device in accordance with any one of the preceding claims. 